Separation of uranium hexafluoride from plutonium hexafluoride by means of selective intercalation in graphite

ABSTRACT

A method is disclosed for the selective separation and sequestering of plutonium from uranium which comprises contacting a gas stream containing uranium hexafluoride [at a partial pressure of 26 torr or less] and plutonium hexafluoride at any pressure with graphite whereby PuF 6  is selectively intercalated into the graphite while UF 6  is left unreacted in the gas stream. Preferably, the contacting of UF 6  and PuF 6  with graphite is conducted in the presence of excess fluorine. 
     In another embodiment the graphite is first reacted with a chemical oxidant. Such pretreatment of the graphite renders it inert to UF 6  intercalation at partial pressures over 26 torr, the pressure above which UF 6  normally intercalates with graphite. Such pretreatment will have less effect on PuF 6  intercalation into the prereacted graphite compound and therefore will permit the separation of UF 6  -PuF 6  by differential intercalation into the prereacted graphite compound to be conducted at higher partial pressures of UF 6 .

BODY OF DISCLOSURE

It has been discovered and forms the basis of this disclosure that at partial pressures of less than 26 torr UF₆ will not intercalate into graphite. However, by virtue of its higher activity, PuF₆ can be expected to react with graphite at threshold partial pressures of less than 26 torr, i.e. a partial pressure at which UF₆ will not intercalate with graphite.

Consequently, based upon this discovery of a threshold pressure for UF₆ intercalation with graphite, a method is disclosed for the selective separation and sequestering of plutonium from uranium which comprises contacting uranium hexafluoride and plutonium hexafluoride with graphite at a UF₆ partial pressure of less than 26 torr whereby PuF₆ is selectively intercalated into the graphite while UF₆ remains unreacted, thereby effecting the desired separation. Preferably, the contacting of UF₆ and PuF₆ with graphite is conducted in the presence of excess fluorine. This is a consequence of the following thermodynamic relationship: considering the reaction

    MF.sub.4 + F.sub.2 ⃡ MF.sub.6,

it shows at room temperature:

    Kp(UF.sub.6) = [UF.sub.6 ]/[F.sub.2 ] = 10.sup.44

    kp(PuF.sub.6) = [PuF.sub.6 ]/ [F.sub.2 ] = 4 × 10.sup.-5

in the graphite compound, both metals (M = Pu and U) will exist in the tetravalent state. This has been demonstrated by NMR and EPR for "graphite/UF₆ ". In view of the fact that pentavalent Pu is not a favorable oxidation state "graphite/PuF₆ " should contain tetravalent Pu. Any increase of fluorine pressure will, thermodynamically, force deintercalation of tetravalent uranium in graphite while leaving tetravalent plutonium untouched. In this manner, graphite becomes a "sink" and a container for the plutonium in the tetravalent oxidation state while rejecting thermodynamic intercalation with UF₆.

Prior to the instant invention, the existence of a threshold UF₆ -graphite intercalation pressure was unknown.

Les Carbones, Vol. II, A. Herold, R. Setton, N. Platzer, eds, Masson et Cie, Paris, 1965 p. 563, references an article by J. Maire in the Proc. Second U.N. Intern. Conf. Peaceful Uses Atomic Energy, 28, and Geneva, 1958, pp 392-395; "Fixation of Bromine and Uranium Hexafluoride on Carbons".

In this Paper, a series of tests were performed on spectroscopically pure material graphites and artificial graphites and medium graphitization.

One test involved introducing UF₆ at high pressure over carbon under vacuum at room temperature. At 300° C, UF₆ was irreversibly desorbed.

In another test, UF₆ was introduced at low pressure to graphite at saturation, natural graphite gained 175% of initial weight, artificial graphite gained 300% of initial weight. Basing weight gain as UF₆ adsorbed, these weights correspond to the formula C₁₆.7 UF₆ and C₉.77 UF₆ respectively. At 250° C, a reversible desorption of 80% of the material was performed in the artificial graphite specimen.

In yet another test, UF₆ was quickly brought into contact with graphite and it was not possible to desorb UF₆ at temperatures of to 300° C.

All of these experiments resulted in intercalation of UF₆ into graphite and would lead one to the inevitable conclusion that intercalation of UF₆ into graphite results at all conditions.

The instant separation method is based upon the discovery that at partial pressures below 26 torr UF₆ will not intercalate into graphite. Figure I is the graphite/UF₆ intercalation isotherm. From this it is clearly seen that a definite threshold partial pressure exists below which the amount of UF₆ which intercalates into graphite is negligible, approaching zero. The threshold pressure is about 26 torr.

By means of this discovery a separation process can be described for the separation of uranium from plutonium involving passing UF₆ and PuF₆ over graphite at a UF₆ partial pressure of less than 26 torr whereby because of the greater reactivity of PuF₆ it will intercalate into graphite while UF₆ will not intercalate because the contacting occurs at less than the threshold pressure.

Preferably, this contacting is performed in the presence of excess fluorine since such excess fluorine will participate in the de-intercalation of any UF₆ which may have sufficient energy to intercalate.

The process of the instant invention may be practiced using any type of graphite from any typical source. Normally graphite has a structure consisting of a hexagonal unit cell with dimensions A_(o) = (2.45 ± 0.10)A C_(o) = (6.7 ± 0.20)A. The only graphite which should be avoided is highly C axis oriented pseudo single crystal pyrolytic graphite.

Alternatively, the graphite may be pretreated with an oxidant which oxidizes the graphite and renders it resistant to UF₆ intercalation. Graphite reacted with CrO₃ /CH₃ CO₂ H and CF₃ CO₂ H/KMnO₄ (not in excess) results in the known intercalation compounds C₁₃ CrO₃ and C₃₀ CF₃ CO₂ H. UF₆ will not intercalate into these compounds at pressures at which it will intercalate into untreated graphite, i.e. partial pressures over 26 torr. Therefore, for situations requiring that graphite be in contact with UF₆ and remain unreacted with UF₆, graphite pretreated with an intercalating oxidant should be used. In this manner, it is possible to produce a graphite material useful in electrochemical reactions and processes. Typical useful oxidants are selected from the group consisting of CuCl₂, CuBr₂ AuCl₃, AlCl₃, GaCl₃, InCl₃, AlBr₃, TiCl₃, ZrCl₄, HfCl₄, SbCl₅, TaCl₅, FeCl₃, CrCl₃, CrO₂ Cl₂, CrO₂ F₂, MoCl₅, α-WCl₆, UCl₄, UO₂ Cl₂, UF₆, ReCl₄, CoCl₃, RuCl₃, RhCl₃, PdCl₄, PtCl₄, IrCl₄, ICl, ICl₃, YCl₃, SmCl₃, CdCl₃, YbCl₃, DyCl₃, EuCl₃, HF, ClF₃, BrF₃, TiF₄, IF₅, AsF₅ , SbF₅, NbF₅, TaF₅, XeF₆, XeOF₄, SbF₃ Cl₂, HgCl₂, MnCl₂, NiCl₂, ZnCl₂, CdCl₂, UCl₅, NbCl₅, MoOCl₄, GaBr₃, AuBr₃, preferably CrO₃ and CF₃ CO₂ H. 

What is claimed is:
 1. A method for selectively separating and sequestering plutonium from uranium comprising contacting a uranium hexafluoride and plutonium hexafluoride mixture with graphite at a UF₆ partial pressure of 26 torr or less whereby PuF₆ is selectively intercalated into the graphite while the graphite is resistant to UF₆ intercalation at such UF₆ partial pressure.
 2. The method of claim 1 wherein the contacting of the mixture of uranium hexafluoride and plutonium hexafluoride with graphite is conducted in the presence of excess fluorine.
 3. The method of claim 1 wherein the graphite has been pretreated with an oxidant rendering said graphite more resistant to UF₆ intercalation.
 4. A method for selectively separating and sequestering plutonium from uranium comprising contacting a mixture of uranium hexafluoride and plutonium hexafluoride with graphite that has been pretreated with a oxidant rendering said graphite more resistant to UF₆ intercalation, at a UF₆ partial pressure of greater than 26 torr, whereby PuF₆ is selectively intercalated into said graphite, said graphite being resistant to UF₆ intercalation at such UF₆ partial pressure.
 5. The method of claim 4 wherein the contacting of the mixture of uranium hexafluoride and plutonium hexafluoride with graphite is conducted in the presence of excess fluorine. 